Transient Melting and Re-Solidification Model of CANDU Core Debris in Severe Accidents

Abstract

This paper describes a computer program simulating the transient behavior of CANDU core debris in the calandria following a severe accident. The program, DEBRIS.MLT, simulates debris heat-u p from a quenched state to the melting point, melting of the debris, superheating of the molten debris above the melting temperature and its eventual cooling and re-solidification and the cooling of the solidified material. While the details of the debris melting process are unknown, the simplified models used in DEBRIS.MLT for the geometric changes occurring in this process provide the appropriate initial and final states for the debris. Simplified but physically reasonable heat transfer models are used in the program. Results obtained by applying the model to a dominant-frequency late core disassembly accident sequence in a CANDU-6 show that the calandria would remain well-cooled throughout the entire transient as long as sufficient water is present in the shield-tank. This conclusion is insensitive to wide variations in the initial porosity and pore-size of the debris and to variations of the thermal properties of the molten material. Failure of the calandria resulting from the boil-off of the shield-tank water would not occur until more than 24 hours after the initiation of the accident, allowing time for operator intervention to mitigate the effects of the accident. Thus, the analysis strengthens the conclusion of earlier studies that the calandria vessel in a CANDU acts as an inherent core-catcher in a severe accident involving late core disassembly and debris melting.